1. Field of the Invention
The present invention relates to a device arranged in a exhaust pipe of a diesel engine for catching and removing particulates in the exhaust gas.
2. Description of the Related Art
Exhaust gas from a diesel engine includes particulates made of carbon etc. There are now strong regulations for eliminattion of these before discharge of the exhaust gas to the atmosphere. In order to eliminate particulates, use is made of a filter made of a ceramic material of, for example, a honeycomb structure having a plurality of axially extending cells with upstream and downstream ends which are alternately opened and closed. The cells have porous walls. Exhaust gas from cells having open upstream ends and closed downstream ends thus passes to the cells having closed upstream ends and open downstream ends. The particulates in the exhaust gas are thus trapped by the porous walls. The porous walls, however, become clogged the longer the filter is used. This causes an increased pressure loss between the inlet and the outlet of the filter. To renew the filter, a heater is provided in the casing of the filter device. This is energized when a renewing operation is necessary and causes the particulates clogged in the filter to be incinerated and eliminated.
During the incinerating process, the temperature at the center portion of the filter is much higher than at the outer peripheral portion. In order to prevent the filter from being thermally damaged, the temperature of the filter must not exceed an upper limit temperature such as 900.degree. C. In this case, the temperature of the filter at the outer peripheral portion is at most 500 .degree. C., which is usually insufficient to fully incinerate the clogged particulates. As a result, along the outer periphery of the filter, particulates remain without being incinerated, which causes the effective area of the filter to be reduced. This reduction in the effective area of the filter increases the speed of the air used for the following incineration process-making it too high so that the ideal incineration process can not be obtained. This in turn increases the amount of particulates remaining without being incinerated. As a result, the more incineration cycles, the larger the amount of the particulates not eliminated.